Where can I find the code behind the Remesh Blocks modifier?
Blender Modifiers are not written in Python but are compiled into Blender and written mostly in C, therefore Remesh won't be found in the scripts folder. The code for Remesh can be found by searching for "MOD_remesh.c"  once you've downloaded the source tree locally. Either download the entire GIT repo, or archived as a zip. Alternatively you can browse the source tree on-line figuring out the path should not be difficult, but here it is anyway:   source/blender/modifiers/intern/MOD_remesh.c

Questions about the C / C++ sources of Blender are off topic for Blender.stackexchange but you can try the developers mailing list or irc freenode #blendercoders.